Feature Request: Support unix sockets

[braun2morrow]

Right now, kamal-proxy can only talk http over TCP/IP. It would be nice to also support http over unix domain sockets, for when the service is located on the same computer. This is supported by most webservers, and has the following advantages:

• Easier to admin: There is no need to allocate port numbers to services and ensure they don't conflict.
• More secure: limited to the local host, and normal file permissions apply to users. In contrast, TCP sockets are accessible to every user, and accidentally binding to :: or 0.0.0.0 may expose them to other hosts.
• More performant: Avoiding the TCP/IP stack reduces kernel overhead.

See e.g. https://krun.pro/nodejs-unix-domain-sockets/ and https://github.com/goldsborough/ipc-bench